1. Field of the Invention
The field of art to which this invention pertains is solid bed adsorptive separation. More specifically, the invention relates to a process for separating an extract component, such as sucrose, from a solution, such as an aqueous solution, where a problem occurs in that the most effective desorbent material also tends to inhibit the selectivity of the adsorbent for the extract component.
2. Prior Art
Sucrose, which is a common form of sugar, is widely used in the food industry. The usual source for this compound is found in the juice of sugar cane, sugar beets and other sucrose-containing materials. After the readily recoverable sucrose has been extracted from these sources, the mother liquors which are generally termed "molasses" will still contain a relatively large amount of sucrose along with other sugars such as glucose, fructose, raffinose, etc. The latter compounds along with salts, amino acids, betaine, pyrollidone, carboxylic acid, etc. constitute crystallization inhibitors which make the recovery of the remaining sucrose difficult to accomplish and thus make the further recovery of the sucrose economically impractical. In addition, the impurities which are present impart a taste to the molasses which renders the same inedible for human consumption.
Sugar beet molasses may contain approximately 50% sucrose and, therefore, it is highly desirable to extract this sucrose from the aforesaid molasses. Inasmuch as hereinbefore set forth, the molasses is bitter to human taste, the residual molasses is used in animal feed or as a fertilizer, and therefore a relatively low sucrose content is an acceptable feature of the molasses. At the present time, there are only a few methods for extracting the sucrose present in molasses from the compounds of the type hereinbefore set forth. One such process which is utilized is the Steffan's process in which the beet molasses is diluted to about 20% solids, refrigerated, and treated with a calcium compound such as calcium oxide. This results in the reaction of the sucrose present with the calcium oxide to form tricalcium sucrate which is an insoluble granular precipitate. This precipitate can then be removed from the diluted molasses solution by filtration followed by washing, to remove adhering impurities. The tricalcium sucrate is returned to the beet processing operation by adding to the incoming hot beet juice. Under such conditions the tricalcium sucrate decomposes, releasing the sucrose to solution so that the calcium oxide has acted as a purification agent. However, a disadvantage which is inherent in the process is that certain impurities are recycled, particularly raffinose, which is a trisaccharide material. With the continual recycling of the tricalcium sucrate, the amount of raffinose present begins to accumulate and, as hereinbefore discussed, will retard the desired crystallization of the sucrose, thus making it necessary to discard a certain amount of circulating molasses from time to time.
In addition to the Steffan process, it is also possible to separate sucrose by utilizing non-continuous chromatographic procedures which employ ion exchange resins to isolate sucrose from the molasses. However, neither of the procedures results in a complete separation of the sucrose even though high purity can be obtained. The processes which effect this separation employ a strong acid, polystyrene ion exchange resin in the alkaline or alkaline earth form and typically are as described by H. J. Hongisto (Technical Department, Finnish Sugar Company Ltd., Kantvik, Finland), "Chromatographic Separation of Sugar Solutions; The Finsugar Molasses Desugarization Process" paper presented to the 23rd Tech. Conf., British Sugar Comp. Ltd., 1976; and by Dr. Mohammad Munir (Central Laboratory, Suddeutsche Zucker AG., 6719 Obrigheim 5, Wormser Str. 1, Germany), "Molasses Sugar Recovery by Liquid Distribution Chromatography"; the International Sugar Journal, 1976, 78, 100-106. A disadvantage which is present in the prior art processes lies in the fact that they require periodic back-flushing and regeneration of the ion exchange resin.
It is also known that certain other solid adsorbents, such as activated carbon selectively adsorb sucrose from an aqueous solution. The sucrose may then be desorbed with alcohol or an alcohol solution. These adsorbents, however, also exhibit a strong affinity for the alcohol and the sucrose is unable to effectively displace the alcohol from the adsorbent in a subsequent adsorption step. Various schemes have been invented whereby the desorbent is flushed from the adsorbent prior to the adsorption step. Although these schemes are successful, a significant disadvantage to them, particularly those adopting a continuous simulated moving bed process, is the requirement of an excessive amount of flush liquid which must eventually be separated from the product streams which requires the expenditure of considerable energy. The simulated moving bed process is described hereinafter in the Detailed Description of the Invention. We have discovered a new process by which the above flushing may be accomplished in a simulated moving bed environment, but with a substantial minimization of the volume of flush liquid required.